The use of shirred casings in the stuffing of meat products constitutes a technological advance which allows a high level of automation in the filling processes. The shirred casings are ready for use without any type of prior manipulation and they can be loaded quickly and simply in the feed units. Due to the great amount of compressed casing that each shirred stick contains, the operation reaches a high degree of autonomy and dead time is minimized.
However, the pressure to which the casings are subjected by being shirred, in combination with certain additives used to provide cohesion in the sticks, confer on the casing an appreciable resistance to unshirring in order to be filled. This resistance is specially important in the case of elastic casings, such as nets, whose resistance to flowing toward the mouth of the filling tube is increased by the friction with the surface of the tube on which they are supported, and which increases as the process of consuming the casing stick advances, since the length of the bare tube on which the casing is rubbing is increasing.
This resistance of the shirred casings to be unshirred for their use interferes with the mechanisms for control of the filling pressure making this increasingly difficult. The control of the pressure at which the filling operation is carried out is responsible for the uniformity in the diameter of the products, for its length in the event of portions of a certain volume being stuffed, and for the absence of air inside the products, causing among other effects, defects of a visual nature and problems with the chemical and biological alteration of the sausages.
The control of the filling pressure is carried out by means of systems which brake the casing at the end of the filling tube and the regulation of which is conditioned by the feeding pressure of the product. In all cases, this control is revealed to be very sensitive to any variation in the resistance to the unshirring of the casing, hence by eliminating this resistance, the variation thereof is eliminated automatically. The best way of doing so, is to continuously provide the regulation system (brake) with the casing completely unshirred.